Jellyfish stings are relatively common problems for people swimming, wading or diving in seawaters. The long tentacles trailing from the jellyfish body can inject you with venom from thousands of microscopic barbed stingers.

Jellyfish stings vary greatly in severity. Most often they result in immediate pain and red, irritated marks on the skin. Some jellyfish stings may cause more whole-body (systemic) illness. And in rare cases jellyfish stings are life-threatening.

Red, brown or purplish tracks on the skin — a "print" of the tentacles' contact with your skin

Itching

Swelling

Tingling and numbness

Throbbing pain that radiates up a leg or an arm

Severe jellyfish stings can affect multiple body systems. These reactions may appear rapidly or several hours after the stings. Signs and symptoms of severe jellyfish stings include:

Nausea and vomiting

Headache

Muscle and joint problems

Weakness and dizziness

Fever

Loss of consciousness

Difficulty breathing

Heart problems

The severity of your reaction depends on:

The type and size of the jellyfish

Your age, size and health, with severe reactions more likely in children and people in poor health

How long you were exposed to the stingers

How much of your skin is affected

When to see a doctor

Seek emergency treatment if:

Stings cover large areas of skin

You have severe symptoms or a serious allergic reaction

Jellyfish tentacles contain microscopic barbed stingers. Each stinger has a tiny bulb that holds venom and a coiled, sharp-tipped tube. The jellyfish uses the venom to protect itself and kill prey.

When you brush against a tentacle, tiny triggers on its surface release the stingers. The tube penetrates the skin and releases venom. It affects the immediate area of contact and may enter the bloodstream.

Jellyfish that have washed up on a beach may still release venomous stingers if touched.

Types of jellyfish

While many types of jellyfish are relatively harmless to humans, some can cause severe pain and are more likely to cause a systemic reaction. These jellyfish cause more-serious problems in people:

Box jellyfish. Also called sea wasps, box jellyfish can cause intense pain. Life-threatening reactions — although rare — are more common with this type. The more dangerous species of box jellyfish are in the warm waters of the Pacific and Indian oceans.

Portuguese man-of-war. Also called bluebottle jellyfish, Portuguese man-of-war jellyfish live mostly in warmer seas. This type has a blue or purplish gas-filled bubble that keeps it afloat on the water and acts as a sail.

Sea nettle. Common in both warm and cool seawaters, sea nettles live along the northeast coast of the United States and are abundant in Chesapeake Bay.

Lion's mane jellyfish. These are the world's largest jellyfish, with a body diameter of more than 3 feet (1 meter). They're most common in cooler, northern regions of the Pacific and Atlantic oceans.

Swimming at times when jellyfish appear in large numbers (a jellyfish bloom)

Swimming or diving in jellyfish areas without protective clothing

Playing or sunbathing where jellyfish are washed up on the beach

Swimming in a place known to have many jellyfish

Possible complications of a jellyfish sting include:

Delayed hypersensitivity reaction, causing blisters, rash or other skin irritations a week or more after the sting

Severe allergic reaction (anaphylaxis), rarely

Infection, rarely

Scarring or skin discoloration, rarely

Most jellyfish stings can be treated by rinsing the area with salt water, applying vinegar or a baking soda paste, and taking a pain reliever.

Someone having a severe reaction to a jellyfish sting needs emergency care that may include:

Cardiopulmonary resuscitation (CPR)

An epinephrine injection, for anaphylactic shock

Life support to stabilize breathing, heart rate and blood pressure

Antivenin medication, if the sting is from a box jellyfish

Pain medicine

Other medical treatments

Other circumstances also may require doctor-supervised treatment:

A rash or other skin reaction due to delayed hypersensitivity may be treated with oral antihistamines or corticosteroids.

A jellyfish sting occurring on or near an eye requires immediate medical care for pain control and a good eye flushing. You will likely be seen by a doctor specializing in eye care (ophthalmologist).

The best treatment for you may depend on the type of jellyfish that stung you. But most stings can be treated with these simple remedies:

Remove stingers. Remove any pieces of jellyfish tentacle in your skin by rinsing the wound with seawater. You can also try gently scraping off the stingers with the edge of an ID card or a credit card. Avoid getting sand on the wound. And don't rinse with fresh water or rub the area with a towel, as these actions may activate more stingers.

Rinse with vinegar or apply a baking soda paste. Rinse the affected area with vinegar for about 30 seconds. Or apply a paste of baking soda and seawater. Each method may deactivate the stingers of some types of jellyfish.

Take a hot shower or apply ice packs. Hot water — as hot as you can tolerate but not above 113 F (45 C) — and ice packs may help ease pain.

Take a pain reliever and apply lotions. Apply calamine lotion or lidocaine to help relieve itching and discomfort.

Remedies to avoid

These remedies are unhelpful or unproved:

Human urine

Meat tenderizer

Solvents, such as formalin, ethanol and gasoline

Pressure bandages

The following tips can help you avoid jellyfish stings:

Wear a protective suit. When swimming or diving in areas where jellyfish stings are possible, wear a wet suit or other protective clothing. Diving stores sell protective "skin suits" or "stinger suits" made of thin, high-tech fabric.

Get information about conditions. Talk to lifeguards, local residents or officials with a local health department before swimming or diving in coastal waters, especially in areas where jellyfish are common.

Avoid water during jellyfish season. Stay out of the water when jellyfish numbers are high.

Be careful where you dive. To avoid stings on the face, don't dive into waters that may have jellyfish.

Use protective lotions. Some clinical evidence shows that lotions such as Safe Sea may result in fewer stings after exposure to jellyfish tentacles.

Leave the water calmly. If you're stung, get out of the water without splashing much. This helps prevent more stingers from releasing venom.

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